(1) Field of the Invention
The present invention relates to an optical transmission system, an optical repeater, and an optical transmission method suitable, for example, for a wavelength division multiplexing (WDM) system employing a short wavelength band.
(2) Description of the Related Art
A WDM system (optical transmission system), which employs a wavelength-multiplexed signal light consisting of a signal light having a plurality of different wavelengths, is being employed to transmit an electrical signal such as voice, data, etc.
In this WDM system, one end office (or one transmission end office) optically modulates and multiplexes electrical signals output from an exchange and transmits the wavelength-multiplexed signal light to an optical repeater of the initial stage. The signal light from the optical repeater is transmitted to a plurality of optical repeaters and is demodulated by the other end office. The demodulated electrical signals are transmitted to other exchanges.
As a conventional method of compensating for an optical signal-to-noise ratio (hereinafter referred to as an optical SN ratio), there is a method of previously emphasizing the optical repeater output of a short wavelength band component by pre-emphasis, or a method of exciting and amplifying this short wavelength band component. The pre-emphasis is the method of equalizing the received optical SN ratios of signal wavelengths by previously reducing at a transmitting side the optical SN ratios of signal wavelengths having less SN ratio degradation.
FIGS. 17A to 17C are diagrams used to explain pre-emphasis, the horizontal axis representing optical wavelengths and the vertical axis representing optical SN ratios. The curve shown in FIG. 17A represents the reception characteristic of a receiving station and shows that the reception characteristic has been degraded at the short wavelength band. The curve shown in FIG. 17B represents a transmission characteristic employing pre-emphasis and shows that a signal light is transmitted after a compensation for transmission degradation is made at a transmitting side. As a result, a flat characteristic is obtained at a receiving side, as indicated by a reception characteristic in FIG. 17C.
If the light power (hereinafter referred to as power) of transmission light to be output from an end office or optical repeater is made too great by the pre-emphasis, a signal light with a plurality of wavelengths will increase the influence of a nonlinear optical effect and develop the nonlinear distortions of cross phase modulation and self-phase modulation, resulting in transmission quality degradation. Because of this, pre-emphasis is often employed in optical auxiliary repeaters.
A general optical repeater is constructed by combination of a centralization amplifier and a distributive amplification. There are two types of centralization amplifiers. One type is an optical fiber, doped with a rare-earth element, such as an erbium-doped fiber (EDF), and light within the optical fiber is excited to optically amplify a transmission signal intensively within the optical fiber. The other type is a dispersion compensation fiber with high nonlinearity in which a transmission signal is intensively amplified by excitation. The centralization amplifier, which employs a fiber doped with a rare-earth element, is used to amplify transmission light by combination of a specific amplification band that a rare-earth element has and excitation light.
The distributive amplification is the method of employing an excitation light source of specific wavelength and the physical properties of an optical fiber employed as a transmission path, and amplifying a signal light distributively by employing optical repeaters between end offices and between optical repeaters and the entire transmission line between the end offices. The distributive amplification normally employs Raman amplification.
The Raman amplification is the technique of performing optical amplification by employing the Raman scattering effect of an optical fiber itself. In general, a Raman amplifier is used for transmitting light, which has a wavelength about 100 nm shorter than that of a signal light, onto a transmission line to amplify the signal light propagating through the transmission line.
In addition, various techniques related to optical repeaters have been proposed (e.g., Japanese Laid-Open Patent Publication No. 2000-330145 (hereinafter referred to as known reference 1), Japanese Laid-Open Patent Publication No. HEI 3-239028 (hereinafter referred to as known reference 2), etc.)
In the case where WDM transmission is performed with a plurality of wavelength bands, the light energy on the short wavelength side excites the light on the long wavelength side by Stimulated Raman Scattering (SRS). As a result, the intensity of received light on the short wavelength side is reduced and the intensity of received light on the long wavelength side is increased.
In addition, in the case of expanding a wavelength band for WDM transmission by an attenuation characteristic that a normal single-mode fiber has, the light loss is minimized at the C band and L band. On the other hand, in the bands on the short wavelength side and long wavelength side with the C and L bands as the center, the light loss is increased. For the light on a short wavelength side such as a S+ band shown in FIG. 3 to be described later, the light loss is further increased.
Therefore, there is a need to improve the intensity of the light on the short wavelength side where loss due to Stimulated Raman Scattering and loss due to optical fibers are added together.
Because of this, in the case where an end office performs pre-emphasis, an output for a short wavelength band is set high, or the characteristic of a centralization amplifier within an optical repeater is determined so that an output for a short wavelength band becomes high, or the intensity of excitation light for distributive amplification with respect to a short wavelength band is increased.
However, if the output of the signal light is made greater, the influence of a nonlinear optical effect will increase in a transmission line, and because of the influence of Stimulated Brillouin Scattering (SBS), an end office, etc., can input only a fixed quantity of power to an optical fiber. This means that a signal light with a predetermined power cannot be transmitted over an optical fiber.
In the case where power input to an optical fiber is great even if the signal light power at a sender is less than a threshold value for Stimulated Brillouin Scattering, there is a possibility that between a plurality of signal lights, noise will occur due to four light wave mixing or inter phase modulation, etc., and will degrade transmission quality.
In the case of performing distributive amplification by excitation light emitted from an optical repeater, light with a wavelength shorter than the short wavelength of a signal light (e.g., light with a wavelength about 100 nm shorter than that of a signal light to be amplified) has to be employed as excitation light. This is because light on a short wavelength side is greatly attenuated by an optical fiber. Because of this, light on a shorter wavelength side than a short wavelength band employed as a signal light is greatly attenuated by an optical fiber, and the intensity of excitation light required for compensating for the attenuation of a signal light in a short wavelength band becomes extremely great.
On the other hand, if the intensity of excitation light is increased, the problem of Stimulated Brillouin Scattering will also occur, as in the case where an end office performs pre-emphasis and the case of a centralization amplifier. Because of this, there is a possibility that the required power cannot be input to an optical fiber.
In the case where the light intensity is made the same as the intensity of light in other wavelength bands by a centralization amplifier and pre-emphasis performed by an end office, the power is also greatly attenuated once within an optical fiber. Because of this, the optical SN ratio is degraded. Thereafter, even if the signal light is amplified, the noise component will become great and it will become difficult to reproduce the signal light.